1. Field of the Invention
The present invention relates to a liquid crystal display device used for display sections of computers and OA apparatuses and the like. More specifically, the present invention relates to a liquid crystal display device which has excellent display characteristics and a high aperture ratio.
2. Description of the Related Art
Conventionally, a liquid crystal display device using an active matrix substrate is known as a display device for computers and OA apparatuses. An example of such a liquid crystal display device using an active matrix substrate is shown in FIG. 26. The active matrix substrate in this example has thin film transistors (hereinbelow referred to as TFTs) as switching elements.
Referring to FIG. 26, TFTs 106 and pixel capacitors 108 are formed in a matrix on a substrate made of glass or the like. A gate electrode of each TFT 106 is connected to a corresponding gate signal line 104, so that the TFT 106 is switched on and off in response to a signal input into the gate electrode via the gate signal line 104. A source electrode of the TFT 106 is connected to a corresponding source signal line 102, so that a video signal is input into the TFT 106. A drain electrode of the TFT 106 is connected to a pixel electrode and one terminal of the corresponding pixel capacitor 108. The other terminal of the pixel capacitor 108 is connected to a corresponding pixel capacitor line 110 and also connected to a counter electrode provided on a substrate facing the active matrix substrate.
FIG. 27 is a plan view of such an active matrix substrate, FIG. 28 is a sectional view taken along line 28--28 of FIG. 27, and FIG. 29 is a sectional view taken along line 29--29 of FIG. 27.
Referring to FIGS. 27 and 28, each pixel of the liquid crystal display device includes the TFT 106 (see FIG. 26), an extended drain electrode 125, a storage capacitor electrode 126, and a pixel electrode 140. Referring to FIG. 29, for each pixel, the gate signal line 104 together with the gate electrode, a gate insulating film 103, a semiconductor layer 134, a channel protection layer 128, an n.sup.+ -Si layer 130 and an ITO (indium tin oxide) film 132 which together constitute the source and drain electrodes, the source signal line 102 made of a metal layer, an interlayer insulating film 136, and the pixel electrode 140 made of a transparent conductive layer are formed in this order on a transparent insulating substrate 120, to form the active matrix substrate. The pixel electrode 140 is connected to the drain electrode of the TFT 106 via a contact hole 142 (see FIG. 28) formed through the interlayer insulating film 136. FIGS. 28 and 29 also show a substrate 122 provided to face the active matrix substrate with a liquid crystal layer 112 interposed therebetween.
In the active matrix substrate with the above configuration, the interlayer insulating film 136 is formed between the gate signal line 104 or the source signal line 102 and the pixel electrode 140. This allows the periphery of the pixel electrode 140 to overlap the signal lines 102 and 104. As a result, a liquid crystal display device with a high aperture ratio can be obtained. Moreover, the overlapping pixel electrode 140 shields an electric field generated due to the potential at the signal lines, effectively suppressing failure in the orientation of liquid crystal molecules.
Referring to FIGS. 28 and 29, a light-shading layer 144, and color layers 146 exhibiting red, blue, or green constituting a color filter are formed on the substrate 122 facing the active matrix substrate with the liquid crystal layer 112 therebetween. A counter electrode 148 and an alignment film 150 are formed in this order on the color filter. Another alignment film 150 is formed on the surface of the active matrix substrate in contact with the liquid crystal layer 112.
FIG. 30A is an enlarged plan view of a portion of FIG. 27 where the gate signal line 104 and the source signal line 102 cross each other. FIG. 30B is a sectional view taken along line 30B--30B of FIG. 30A, showing the overlap portions of the pixel electrodes 140 on the source signal line 102.
Referring to FIG. 30A, the vertically adjacent pixel electrodes 140 overlap the corresponding gate signal line 104 by overlap widths dg1 and dg2, while the horizontally adjacent pixel electrodes 140 overlap the corresponding source signal line 102 by overlap widths ds1 and ds2. These overlap widths are generally determined in consideration of the processing precision of the gate signal lines 104 and the source signal lines 102 which serve as light-shading films, the overlap precision of the pixel electrodes 140 on the gate signal lines 104 and the source signal lines 102, and the processing precision of the pixel electrodes 140. Conventionally, the pixel electrodes 140 overlap the gate signal lines 104 and the source signal lines 102 so that the overlap widths dg1 and dg2 are equal to each other and the overlap widths ds1 and ds2 are equal to each other.
The liquid crystal display device where the pixel electrodes overlap the signal lines as described above causes no problem as far as it is driven by a flame inversion driving method. However, when such a liquid crystal display device is driven by a gate line inversion driving method, a source line inversion driving method, or a dot inversion driving method, the following problem arises. That is, referring now to FIG. 30B, the orientation of liquid crystal molecules 152a is disturbed due to an electric field generated between the adjacent pixel electrodes, generating a reverse tilt domain having liquid crystal molecules 152b which have a reverse pretilt angle, i.e., are oriented in the opposite direction of an orientation direction D.sub.1 (see FIG. 30A) of the liquid crystal molecules 152a. The generation of such a reverse tilt domain causes light leakage and thus eminently degrades the display characteristics of the resultant liquid crystal display device.
In order to prevent light leakage of the liquid crystal display device due to the disturbance of the orientation of liquid crystal molecules, increasing the overlap widths of the pixel electrodes on the gate signal lines and the source signal lines is known. Increasing the overlap widths, however, causes another problem of increasing the occupation of the light-shading portions in the liquid crystal display device and thus decreasing the aperture ratio.
Also known is a liquid crystal display device where each pixel is divided into two portions having different orientation directions D.sub.1 and D.sub.2 of liquid crystal molecules as shown in FIGS. 31A to 31C. FIG. 31A is a plan view of a portion of such a liquid crystal display device where a gate signal line 104 and a source signal line 102 cross each other. FIG. 31B is a sectional view taken along line 31B--31B of FIG. 31A, and FIG. 31C is a sectional view taken along line 31C--31C of FIG. 31A.
In such a liquid crystal display device, also, pixel electrodes conventionally overlap signal lines so that overlap widths dg1 and dg2 are equal to each other and overlap widths ds1 and ds2 are equal to each other as shown in FIG. 31A. This causes no problem as far as the liquid crystal display device is driven by a flame inversion driving method. However, as in the above case, when it is driven by a gate line inversion driving method, a source line inversion driving method, or a dot inversion driving method, the following problem arises. That is, the orientation of liquid crystal molecules 152a are disturbed due to an electric field generated between the adjacent pixel electrodes, generating a reverse tilt domain having liquid crystal molecules 152b which have a reverse pretilt angle as shown in FIGS. 31B and 31C. This causes light leakage and thus eminently degrades the display characteristics of the resultant liquid crystal display device.
In this case, as in the above case, the overlap widths of the pixel electrodes on the gate signal lines and the source signal lines may be increased to prevent light leakage due to the disturbance of the orientation of liquid crystal molecules. However, this causes another problem of increasing the occupation of the light-shading portions in the liquid crystal display device and thus decreasing the aperture ratio.
Referring to FIGS. 27 and 28 again, a reverse tilt domain is also generated in a region at and around each contact hole 142 as indicated by the reference numeral 154. Such a reverse tilt domain tends to be generated especially when the angle of the inner wall of the contact hole 142 with respect to the substrate surface exceeds 45.degree.. Light leakage may occur even when the liquid crystal layer 112 is switched from a light-transmitting state to a light-shading state.
In order to prevent light leakage at and around the contact hole, it is known to shade the region at and around the contact hole 142 by using a light-shading material for the storage capacitor electrode 126 above which the contact hole 142 is formed. For complete light-shading, however, the size of the storage capacitor electrode 126 needs to be sufficiently large. This causes a problem of substantially decreasing the display area of each pixel and thus decreasing the aperture ratio of the resultant liquid crystal display device.
Japanese Laid-open Patent Publication No. 5-249494 discloses a method for suppressing the generation of a reverse tilt domain in an active matrix liquid crystal display device. In the disclosed method, the angles of sloped steps between pixel electrodes and gate and source signal lines with respect to a substrate surface are set at 60.degree. or less, thereby preventing a generation of disclination lines on a display screen.
According to the above method, however, as described in the Publication, a satisfactory result is not obtainable when the difference between the pixel electrodes and the gate and signal lines exceeds 2 .mu.m. As for the contact hole 142 shown in FIG. 28, for example, a reverse tilt domain is generated at and around the contact hole 142 when the difference (corresponding to the thickness of the interlayer insulating film 136) exceeds 2 .mu.m. Therefore, in order to apply the disclosed method to the contact hole 142, the thickness of the interlayer insulating film 136 should be 2 .mu.m or less.
The interlayer insulating film 136, on the other hand, needs to be sufficiently thick to have a flat surface, which is required to flatten the alignment film 150 to be formed above the interlayer insulating film 136 in contact with the liquid crystal layer 112. Accordingly, it is substantially difficult to set the thickness of the interlayer insulating film 136 at 2 .mu.m or less. The above disclosed method is therefore not applicable to the region including the contact hole.